System and method for recovering metal from metal-containing waste liquid

ABSTRACT

Disclosed is a system for recovering metal from metal-containing waste liquid. The system includes a waste liquid storage unit, an extraction unit, and an electrolysis unit. The waste liquid storage unit is configured to store a metal-containing waste liquid. The extraction unit is in fluid connection with the waste liquid storage unit and includes an extraction device and a back-extraction device. The extraction device is configured to collect a target metal ion present in the metal-containing waste liquid, and the back-extraction device is configured to back-extract the target metal ion into a back-extracting liquid to form a metal compound. The electrolysis unit is in fluid connection with the waste liquid storage unit and the extraction unit, configured to reduce the target metal ion to a solid metal, or to dissociate the metal compound and deposit a solid metal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The instant disclosure relates to technologies for the recovery of metal resources, in particular, to a system and method for recovering metal from metal-containing waste liquid.

2. Description of Related Art

With the rapid advancement of the electronics industry, electronic products such as home appliances and 3C products are manufactured in large quantities, sold, and widely used. Generally, waste liquids (e.g. acid waste liquids) are produced by processes for manufacturing home appliances and 3C products.

Acid waste liquids usually include recyclable acids such as phosphoric acid, acetic acid, nitric acid, and sulfuric acid and/or useful metal components. There are many ways to recover acids and/or metal components. For example, metal ions in waste liquids can be reduced into metals by electroplating. However, the recovery efficiency and quality of metals must be influenced by concentrations of metal ions in the waste liquids. Hence, concentrations of metal ions in the waste liquids need to be monitored and accumulated to an acceptable level. This results in an increase of both manpower costs and quality control costs. In addition, acid waste liquids having a low boiling point under atmospheric pressure, i.e., nitric acid, need to be recovered by distillation. Acid waste liquids having a high boiling point under atmospheric pressure, i.e., sulfuric acid, need additional recovery apparatus. Therefore, the complexity of conventional recovery processes and their recovery costs are increased due to the use of different recovery systems of acid waste liquids and metal components.

In summary, there is an urgent need of a recovery system with high integration of acid waste liquids and metal components which can be used to reduce the complexity of conventional recovery processes and their recovery costs to solve the problems of prior art.

SUMMARY OF THE INVENTION

For the purposes of comprehensive recovery of metal resources and reduction of environmental burden, the object of the instant disclosure is to provide a system and method to recover a metal component or components existing in waste water, and the resulting metal or metals with high purity can be reused.

In order to achieve the aforementioned objects, according to an embodiment of the instant disclosure, the system for recovering metal from metal-containing waste liquid comprises a waste liquid storage unit, an extraction unit, and an electrolysis unit. The waste liquid storage unit is configured to store a metal-containing waste liquid. The extraction unit is in fluid connection with the waste liquid storage unit and includes an extraction device and a back-extraction device. The extraction device is configured to collect a target metal ion present in the metal-containing waste liquid, and the back-extraction device is configured to back-extract the target metal ion into a back-extracting liquid to form a metal compound. The electrolysis unit is in fluid connection with the waste liquid storage unit and the extraction unit, configured to reduce the target metal ion to a solid metal, or to dissociate the metal compound and deposit a solid metal.

In one respect, the method for recovering metal from metal-containing waste liquid comprises: extracting a target metal ion from a metal-containing waste liquid; back-extracting the target metal ion into a back-extracting liquid to form a metal compound; and electrolyzing the back-extracting liquid to dissociate the metal compound and deposit a solid metal.

In another respect, the method for recovering metal from metal-containing waste liquid comprises: directing a metal-containing waste liquid to a waste liquid storage unit for temporary storage; monitoring concentrations of metal ions in the metal-containing waste liquid; electrolyzing the metal-containing waste liquid to reduce a target metal ion at relatively high concentration into a solid metal according the monitoring result; and directing the electrolyzed metal-containing waste liquid back to the waste liquid storage unit.

In still another respect, the method for recovering metal from metal-containing waste liquid comprises: providing a metal-containing waste liquid containing first and second metal ions, wherein the concentration of the first metal ion is greater than that of the concentration of the second metal ion; extracting the second metal ion from the metal-containing waste liquid; electrolyzing the extracted metal-containing waste liquid to reduce the first metal ion into a first solid metal and back-extracting the second metal ion into a back-extracting liquid to form a metal-containing back-extracting liquid; and electrolyzing the metal-containing back-extracting liquid to reduce the second metal ion into a second solid metal.

Based on the above, the waste liquid storage unit, the extraction unit, and the electrolysis unit are fluidly connected together through specific pipelines, such that the system can begin to execute the process of sequentially extracting, back-extracting and electrolyzing or the process of sequentially electrolyzing, extracting, and back-extracting. Thereby, the recovery efficiency of metal resource can be remarkably increased. Moreover, the lifetimes of the electrode plates within the electrolysis unit and the resin within the extraction device can be extended, and the cost of electricity and supplies can be reduced.

In order to further appreciate the characteristics and technical contents of the instant disclosure, references are hereunder made to the detailed descriptions and appended drawings in connection with the instant disclosure. However, the appended drawings are merely shown for exemplary purposes, rather than being used to restrict the scope of the instant disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a system for recovering metal from metal-containing waste liquid according to a first embodiment of the present invention.

FIG. 2 is an architecture diagram of the system S according to the first embodiment of the present invention.

FIG. 3 is a block diagram showing a system for recovering metal from metal-containing waste liquid according to a second embodiment of the present invention.

FIG. 4 is an architecture diagram of the system S according to the second embodiment of the present invention.

FIG. 5 a flowchart of a method for recovering metal from metal-containing waste liquid according to a preferred embodiment of the present invention.

FIG. 6 a flowchart of a method for recovering metal from metal-containing waste liquid according to another preferred embodiment of the present invention.

FIG. 7 a flowchart of a method for recovering metal from metal-containing waste liquid according to still another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The aforementioned illustrations and following detailed descriptions are exemplary for the purpose of further explaining the scope of the instant disclosure. Other objectives and advantages related to the instant disclosure will be illustrated in the subsequent descriptions and appended drawings.

First Embodiment

Please refer to FIG. 1. This is a block diagram showing a system for recovering metal from metal-containing waste liquid according to the first embodiment of the present invention. The system S includes a waste liquid storage unit 1, an extraction unit 2, and an electrolysis unit 3 fluidly connected to each other. The waste liquid storage unit 1 is configured to store a metal-containing waste liquid. The extraction unit 2 is configured to extract and back-extract a target metal component in the metal-containing waste liquid. The electrolysis unit 3 is configured to reduce the target metal component into a solid metal, or to dissociate the target metal component and deposit a solid metal.

Please note that the system S is applicable to any waste liquid, such as noble metal waste liquid, solder stripping waste liquid, acidic etching waste liquid, alkaline etching waste liquid, microetching waste liquid, and electroplating waste liquid, including only one metal component or plural metal components. The resulting waste liquids can meet the national wastewater effluent discharge standards.

For the instant embodiment, the waste liquid storage unit 1 is a waste liquid storage tank. The extraction unit 2 includes an extraction device 21 and a back-extraction device 22 fluidly connected to each other. The extraction device 21 is a resin extraction column, and the back-extraction device 22 is an extraction tank. Preferably, the main bodies of the waste liquid storage tank, the resin extraction column, and the extraction tank mentioned above are made from corrosion-resistant material (e.g., stainless steel or corrosion-resistant polymer).

The waste liquid storage unit 1 includes at least one concentration detector 11 for monitoring concentrations of metal ions in the metal-containing waste liquid. For example, an etching waste liquid may contain a large amount of copper ion and small amount of other metal ions (e.g., plumbum, zinc, and aluminum ions). In the waste liquid storage unit 1, there can be arranged an analyser for determining target metal ions and their concentration.

The extraction device 21 includes a resin that is capable of absorbing target metal ions. Specific preferred examples of the resin include chelate resins sold under the designation DIAION, such as CR11 and CR20 from Mitsubishi Kasei Corp. Please note that the resin within the extraction device 21 can be adjusted as required according to a metal component or components in the waste liquid. In more detail, CR11 can be used to absorb metal ions by the extraction behavior of diglycolamidic acid resin. CR11 has a high selectivity for the adsorption of divalent ions, in particular to transition metals such as copper and iron. CR20 can be used to absorb metal ions by the extraction behavior of polymerized amino resin. CR20 has a high selectivity for the adsorption of heavy metal ions.

Please refer to FIG. 2, which is an architecture diagram of the system S. It should be noted that the metal-containing waste liquid can be continuously delivered, circulated, and/or discharged between the waste liquid storage unit 1, the extraction unit 2, and the electrolysis unit 3 through specific pipelines. To firstly describe the connection relationship between the waste liquid storage unit 1 and the extraction unit 2, a top discharge end 1 a of the waste liquid storage unit 1 is in fluid connection with a top inlet end 21 a of the extraction device 21 through a liquid delivery pipe (not shown). Accordingly, the metal-containing waste liquid can be delivered to the extraction device 21 from the waste liquid storage unit 1 when concentrations of metal ions accumulate to an acceptable level. After that, the extraction device 21 can absorb metal ions and decrease concentrations of metal ions in the metal-containing waste liquid.

The extraction device 21 is in fluid connection with the back-extraction device 22 through a circulation pipeline 23. The circulation pipeline 23 includes a first liquid delivery pipe 231 and a second liquid delivery pipe 232, a top discharge end 21 b of the extraction device 21 is in fluid connection with a top inlet end 22 a of the back-extraction device 22 through the first liquid delivery pipe 231, and a bottom inlet end 21 c of the extraction device 21 is in fluid connection with a bottom discharge end 22 b of the back-extraction device 22 through the second liquid delivery pipe 232. Accordingly, the back-extracting liquid can be circulated between the extraction device 21 and the back-extraction device 22 as many times as is necessary, and contact with metal ions to form a metal-containing back-extracting liquid.

To further describe the connection relationship between the waste liquid storage unit 1, the extraction unit 2, and the electrolysis unit 3, and the electrolysis unit 3, the electrolysis unit 3 is in fluid connection with the waste liquid storage unit 1 and the circulation pipeline 23 through an inlet pipeline 31 thereof. Specifically, the inlet pipeline 31 of the electrolysis unit 3 includes a waste liquid delivery pipe 311, a back-extracting liquid discharge pipe 312, and an inlet pipe 313. A bottom discharge end 1 b of the waste liquid storage unit 1 is in fluid connection with one end of the inlet pipe 313 through the waste liquid delivery pipe 311, the other end of the inlet pipe 313 is connected to a top inlet end 3 a of the electrolysis unit 3, and the back-extracting liquid discharge pipe 312 is connected between a branch point X1 of the second liquid delivery pipe 232 and the inlet pipe 313. Accordingly, the system S can deliver the metal-containing waste liquid or the metal-containing back-extracting liquid to the electrolysis unit 3 according to metal concentration levels in liquids. Thereby, the electrolysis unit 3 can reduce the metal component to solid metal using an electrolytic process.

The system S can be integrated with water pollution prevention management. For the instant embodiment, the waste liquid storage unit 1, the extraction unit 2, and the electrolysis unit 3 are in fluid connection with a treatment facility 4 through a waste liquid treating pipeline 41. Specifically, the waste liquid treating pipeline 41 includes a first waste liquid discharge pipe 411, a second waste liquid discharge pipe 412, a third waste liquid discharge pipe 413, and at least one forth waste liquid discharge pipe 414. Another bottom discharge end 1 c of the waste liquid storage unit 1 is in fluid connection with the treatment facility 4 through the first waste liquid discharge pipe 411. The second waste liquid discharge pipe 412 is connected between a branch point X2 of the waste liquid delivery pipe 311 and the first waste liquid discharge pipe 411. Another bottom discharge end 22 c of the back-extraction device 22 is in fluid connection with the first and second waste liquid discharge pipes 411, 412 through the third waste liquid discharge pipe 413. At least one discharge end 3 b of the electrolysis unit 3 is in fluid connection with the first waste liquid discharge pipe 411 through the at least one fourth waste liquid discharge pipe 414. The extracted or the electrolyzed waste liquid can be directly delivered to the treatment facility 4 for water pollution abatement.

Referring again to FIG. 2, the system S further includes an air source 5 that is in fluid connection with an air intake 3 c of the electrolysis unit 3. Accordingly, the airflow provided by the air source 5 can be introduced to the electrolysis unit 3 by pulsating in the electrolysis of the extracted or electrolyzed waste liquid to enhance the reduction efficiency. For example, the pulsating air flow can be used to enhance the conversion of copper to high-priced copper ion.

To achieve the automatic control of delivery, circulation, and discharge of the processed waste liquid, there is arranged at least one pump and control valve on each of the circulation pipeline 23, the inlet pipeline 31, and the waste liquid treating pipeline 41. The pump can be a high pressure pump, and the control valve can be a chemical valve. Accordingly, the pumps and control valves can be controlled by a controller (not shown) to adjust the flow volume and direction of flow.

Please note that the system S shown in FIG. 2 includes only one extraction unit 2, but in various embodiments the system S can include a plurality of extraction units 2. Embodiments are not limited to any particular number of extraction units 2.

Second Embodiment

Please refer to FIG. 3. This is a block diagram showing a system for recovering metal from metal-containing waste liquid according to the second embodiment of the present invention. Compared with the first embodiment, the system S′ further includes a conditioning unit 6 that is in fluid connection with the back-extraction device 22 of the extraction unit 2 and the electrolysis unit 3. The conditioning unit 6 is configured to recycle the electrolyzed back-extracting liquid discharged from the electrolytic unit 3, and then to supply the conditioned back-extracting liquid to the back-extraction device 22. In addition, the air source 5 is in fluid connection with an air intake 3 c of the electrolysis unit 3 and an air intake 62 e of the conditioning tank 62 through an air supply pipeline 51. Thereby, the airflow provided by the air source 5 can be introduced to the electrolysis unit 3 and the conditioning tank 62 by pulsating.

Please refer to FIG. 4. This is an architecture diagram of the system S′. The conditioning unit 6 includes a chemical feeding tank 61 and a conditioning tank 62. Specifically, a bottom discharge end 61 a of the chemical feeding tank 61 is in fluid connection with a first top inlet end 62 a of the conditioning tank 62 through a chemical delivery pipe 611. A bottom discharge end 62 b of the conditioning tank 62 is in fluid connection with another top inlet end 22 d of the back-extraction device 22 through a chemical supply pipe 612. For the purpose of reusing chemicals, the waste liquid treating pipeline 41 further includes a waste liquid recycling pipe 415. The at least one bottom discharge end 3 b of the electrolysis unit 3 is in fluid connection with a second top inlet end 62 c of the conditioning tank 62.

The system S′ further includes a water source 7 that is in fluid connection with a third top inlet end 62 d of the conditioning tank 62 through a water supply pipeline 71. Accordingly, the waterflow provided by the water source 7 can be directed to the conditioning tank 62 to adjust pH value of a new back-extracting liquid. For example, the new back-extracting liquid may be a pH-adjusted mixture of a portion of the back-extracting liquid recovered by the system S′, an unused chemical provided by the chemical feeding tank 61 and a proper amount of water.

The benefits, features and advantages of the system S (S′) are mentioned above. The following will describe a method for recovering metal from metal-containing waste liquid which can be applied to the system S (S′).

Please refer to FIG. 5, which is a flow chart of the method according to a preferred embodiment of the instant disclosure. The method includes: extracting a target metal ion from a metal-containing waste liquid (step S100); back-extracting the target metal ion into a back-extracting liquid to form a metal compound (step S1002); and electrolyzing the back-extracting liquid to dissociate the metal compound and deposit a solid metal (step S104).

Refer to FIGS. 2 and 4. In step S100, the metal-containing waste liquid stored in the waste liquid storage unit 1 is delivered to the extraction device 21 for extraction of a target metal-ion when said metal ion is greater than a predetermined amount. For example, said metal ion can be bonded/absorbed up to a specific polymer (e.g., chelating resin). In step S102, the back-extracting liquid is circulated between the extraction device 21 and the back-extraction device 22 through the first and second liquid delivery pipes 231, 232 as many times as is necessary, and contacts with metal ions to form a metal-containing back-extracting liquid. For example, the back-extracting liquid provided by the back-extraction device 22 includes diluted hydrochloric acid which has a high selectivity for the adsorption of copper, cobalt, zinc, manganese ions and so on. In step S104, the metal-containing back-extracting liquid is delivered to the electrolysis unit 3 through the back-extracting liquid discharge pipe 312 and the inlet pipe 313, and to dissociate and deposit a target solid metal using an electrolytic process.

Please refer to FIG. 6, which is a flow chart of the method according to another preferred embodiment of the instant disclosure. The method includes: directing a metal-containing waste liquid to a waste liquid storage unit for temporary storage (step S200); monitoring concentrations of metal ions in the metal-containing waste liquid (step S202); electrolyzing the metal-containing waste liquid to reduce a target metal ion at relatively high concentration into a solid metal according the monitoring result (step S204); and directing the electrolyzed metal-containing waste liquid back to the waste liquid storage unit (step S206).

Referring to FIGS. 2 and 4. In step S200 and step S202, the metal-containing waste liquid can be directed to the waste liquid storage unit 1 from a discharge end of a processing device end (not shown). After that, metal concentration present in the metal-containing waste liquid can be detected by a concentration detector 11. In step S204, when a target metal ion (e.g. copper ion) is greater than a predetermined amount (e.g. 5 wt %±0.5 wt %), the metal-containing back-extracting liquid is delivered to the electrolysis unit 3 through the waste liquid delivery pipe 311 and the inlet pipe 313. After that, a metal compound (e.g. copper-based compound) within the metal-containing back-extracting liquid can be dissociated to deposit a target solid metal (e.g. copper) using an electrolytic process. In step S206, the electrolyzed metal-containing waste liquid with very low levels of metal ions is delivered back to the waste liquid storage unit 1 for accumulation of metal ions in the wastewater. For example, the electrolyzed metal-containing liquid may have a very low levels of metal ions such as gold or silver.

Please note that the system S (S′) will begin to execute step S100 to step S104 when target metal ions (e.g. copper ions) are lower than a predetermined amount (e.g. 5 wt %±0.5 wt %). On the other hand, taking copper ion for example, if the concentration of copper ions present in the metal-containing waste liquid is greater than 5 wt %±0.5 wt % or the concentration of gold or silver ions present in the metal-containing waste liquid is lower than 3 wt %±0.5 wt %, the system S (S′) will begin to execute step S200 to step S206. Accordingly, the lifetimes of the substrates (i.e. electrode plates) within the electrolysis unit 3 and the resin within the extraction device 21 can be extended, and the cost of electricity can be reduced.

Please refer to FIG. 7, which is a flow chart of the method according to still another preferred embodiment of the instant disclosure. The method includes: providing a metal-containing waste liquid containing first and second metal ions, wherein the concentration of the first metal ion is greater than that of the concentration of the second metal ion (step S300); extracting the second metal ion from the metal-containing waste liquid (step S302); electrolyzing the extracted metal-containing waste liquid to reduce the first metal ion into a first solid metal and back-extracting the second metal ion into a back-extracting liquid to form a metal-containing back-extracting liquid (step S304); and electrolyzing the metal-containing back-extracting liquid to reduce the second metal ion into a second solid metal (step S306).

In step S300, the metal-containing waste liquid containing first and second metal ions can be directed to the waste liquid storage unit 1 from a discharge end of a processing device end (not shown). The first metal ions are different from the second metal ions. For example, the first metal ions at a relatively high concentration are copper ions, and the second metal ions at a relatively low concentration are gold or silver ions. In step S302, the metal-containing waste liquid is delivered to the extraction device 21 for extraction of the first metal ions. For example, the first metal ions (i.e. copper ions) can be bonded/absorbed up to a specific polymer, such that the metal-containing waste liquid contains only second metal ions (i.e. gold or silver ions). In step S304, the extracted metal-containing waste liquid is delivered to the electrolysis unit 3 to reduce the first metal ions into first solid metals (i.e. copper) using an electrolytic process. At the same time, the back-extracting liquid is circulated between the extraction device 21 and the back-extraction device 22 to absorb the second metal ions to form a metal-containing back-extracting liquid. In step S306, the metal-containing back-extracting liquid is delivered to the electrolysis unit 3 to reduce the second metal ions into second solid metals (i.e. gold or silver) using an electrolytic process. Accordingly, the instant method can be used to recover any waste liquid containing at least two metal components. Please note that the number of the extraction units 2 of the system S (S′) can be adjusted according to the number of metal components presented in the waste liquid. Preferably, the number of the extraction units 2 is larger than the number of metal components plus one. Thereby, the recovery efficiency of metal resources can be remarkably increased.

Based on the above, compared with the conventional technologies for treating metal-containing waste liquid, the system/method of the present invention has the following advantages:

The waste liquid storage unit, the extraction unit, and the electrolysis unit are fluidly connected together through specific pipelines, such that the system can begin to execute the process of sequentially extracting, back-extracting and electrolyzing or the process of sequentially electrolyzing, extracting, and back-extracting. Thereby, the recovery efficiency of metal resources can be remarkably increased. In addition, the lifetimes of the electrode plates within the electrolysis unit and the resin within the extraction device can be extended, and the cost of electricity and supplies can be reduced.

Moreover, by the arrangement of the conditioning unit, the system can provide a new back-extracting liquid which represents a pH-adjusted mixture of a portion of the back-extracting liquid, an unused chemical, and a proper amount of water. Thereby, the cost of back-extracting liquid can be reduced.

In addition, the method can recover a metal component or components existing in waste water produced from semiconductor elements, passive elements, PCB, and LCD related processes. The resulting waste water having a very low level of metal ions can meet international wastewater discharge standards, and the resulting metal or metals with high purity can be reused.

The descriptions illustrated supra set forth simply the preferred embodiments of the instant disclosure; however, the characteristics of the instant disclosure are by no means restricted thereto. All changes, alterations, or modifications conveniently considered by those skilled in the art are deemed to be encompassed within the scope of the instant disclosure delineated by the following claims. 

What is claimed is:
 1. A system for recovering metal from metal-containing waste liquid, comprising: a waste liquid storage unit configured to store a metal-containing waste liquid; an extraction unit in fluid connection with the waste liquid storage unit, including an extraction device and a back-extraction device, wherein the extraction device is configured to collect a target metal ion present in the metal-containing waste liquid, and the back-extraction device is configured to back-extract the target metal ion into a back-extracting liquid to form a metal compound; and an electrolysis unit in fluid connection with the waste liquid storage unit and the extraction unit, configured to reduce the target metal ion to a solid metal, or to dissociate the metal compound and deposit a solid metal.
 2. The system according to claim 1, wherein the extraction device is in fluid connection with the back-extraction device through a circulation pipeline, and the electrolysis unit is in fluid connection with the waste liquid storage unit and the circulation pipeline through an inlet pipeline thereof.
 3. The system according to claim 2, wherein a top discharge end of the waste liquid storage unit is in fluid connection with an top inlet end of the extraction device, the circulation pipeline includes a first liquid delivery pipe and a second liquid delivery pipe, a top discharge end of the extraction device is in fluid connection with an top inlet end of the back-extraction device through the first liquid delivery pipe, and a bottom inlet end of the extraction device is in fluid connection with a bottom discharge end of the back-extraction device through the second liquid delivery pipe.
 4. The system according to claim 3, wherein the inlet pipeline of the electrolysis unit includes a waste liquid delivery pipe, a back-extracting liquid discharge pipe, and an inlet pipe, a bottom discharge end of the waste liquid storage unit is in fluid connection with one end of the inlet pipe through the waste liquid delivery pipe, the other end of the inlet pipe is connected to a top inlet end of the electrolysis unit, and the back-extracting liquid discharge pipe is connected between a branch point of the second liquid delivery pipe and the inlet pipe.
 5. The system according to claim 1, wherein the waste liquid storage unit, the extraction unit, and the electrolysis unit are in fluid connection with a treatment facility through a waste liquid treating pipeline.
 6. The system according to claim 5, wherein the waste liquid treating pipeline includes a first waste liquid discharge pipe, a second waste liquid discharge pipe, a third waste liquid discharge pipe, and at least one fourth waste liquid discharge pipe, another bottom discharge end of the waste liquid storage unit is in fluid connection with the facility management through the first waste liquid discharge pipe, the second waste liquid discharge pipe is connected between a branch point of the waste liquid delivery pipe and the first waste liquid discharge pipe, another bottom discharge end of the back-extraction device is in fluid connection with the first and second waste liquid discharge pipes through the third waste liquid discharge pipe, and at least one discharge end of the electrolysis unit is in fluid connection with the first waste liquid discharge pipe through the at least one fourth waste liquid discharge pipe.
 7. The system according to claim 1, further comprising a conditioning unit in fluid connection with the back-extraction device and the electrolysis unit, configured to recover the electrolyzed back-extracting liquid discharged from the electrolytic unit and supply the conditioned back-extracting liquid to the back-extraction device.
 8. The system according to claim 7, wherein the conditioning unit includes a chemical feeding tank and a conditioning tank, a bottom discharge end of the chemical feeding tank is in fluid connection with a first top inlet end of the conditioning tank through a chemical delivery pipe, and a bottom discharge end of the conditioning tank is in fluid connection with another top inlet end of the back-extraction device through a chemical supply pipe.
 9. The system according to claim 8, wherein the waste liquid treating pipeline further includes a waste liquid recycling pipe, a bottom discharge end of the electrolysis unit is in fluid connection with a second top inlet end of the conditioning tank.
 10. The system according to claim 9, further comprising a water source and an air source, the water source is in fluid connection with a third top inlet end of the conditioning tank through a water supply pipeline, and the air source is in fluid connection with air intakes of the electrolysis unit and the conditioning tank through an air supply pipeline respectively.
 11. The system according to claim 1, wherein the waste liquid storage unit includes a concentration detector for detecting metal concentration present in the metal-containing waste liquid.
 12. A method for recovering metal from metal-containing waste liquid, comprising the following steps: extracting a target metal ion from a metal-containing waste liquid; back-extracting the target metal ion into a back-extracting liquid to form a metal compound; and electrolyzing the back-extracting liquid to dissociate the metal compound and deposit a solid metal.
 13. The method according to claim 12, before the step of extracting a target metal ion from a metal-containing waste liquid, further comprising: directing the metal-containing waste liquid to a waste liquid storage unit for temporary storage.
 14. The method according to claim 13, wherein the step of directing the metal-containing waste liquid to a waste liquid storage unit for temporary storage further comprises: monitoring concentrations of metal ions in the metal-containing waste liquid.
 15. The method according to claim 14, between the step of extracting a target metal ion from a metal-containing waste liquid and the step of extracting a target metal ion from a metal-containing waste liquid, further comprising: electrolyzing the metal-containing waste liquid to reduce a target metal ion at relatively high concentration into a solid metal according the monitoring result; and directing the electrolyzed metal-containing waste liquid back to the waste liquid storage unit.
 16. The method according to claim 12, wherein the back-extracting liquid includes hydrochloric acid.
 17. A method for recovering metal from metal-containing waste liquid, comprising the following steps: providing a metal-containing waste liquid containing first and second metal ions, wherein the concentration of the first metal ion is greater than that of the concentration of the second metal ion; extracting the second metal ion from the metal-containing waste liquid; electrolyzing the extracted metal-containing waste liquid to reduce the first metal ion into a first solid metal and back-extracting the second metal ion into a back-extracting liquid to form a metal-containing back-extracting liquid; and electrolyzing the metal-containing back-extracting liquid to reduce the second metal ion into a second solid metal.
 18. The method according to claim 17, wherein the step of providing a metal-containing waste liquid containing first and second metal ions further comprises: directing the metal-containing waste liquid containing first and second metal ions to a waste liquid storage unit for temporary storage.
 19. The method according to claim 18, wherein the first metal ions at a relatively high concentration are copper ions and the second metal ions at a relatively low concentration are gold or silver ions.
 20. The method according to claim 17, wherein the back-extracting liquid includes hydrochloric acid. 